1. Field of the Invention
This invention relates generally to lift trucks and more particularly to an extensible mast structure for use on a lift truck.
More particularly, the invention relates to an extensible mast structure for an industrial lift truck. The mast structure includes a fixed mast section, at least one movable mast section mounted for vertical movement relative to the fixed mast section and a load carrying device which is vertically movable along the uppermost movable mast section. The extensible mast structure includes a lift arrangement for moving the load carrying device and for extending the mast structure.
2. Description of the Prior Art
It is known to provide a primary jack for moving a load carrying device along a movable mast, and a secondary jack for extending the mast structure. The two jacks have selected operational cross-sectional areas so that the primary jack is sequenced to raise the load carrying device before the secondary jack extends the mast structure. This means that full-free lift of the load carrying device is followed by the extension of the mast structure which continues to raise the load carrying device. Although there is no evident operational time lag between the end of the extension of the primary jack and the beginning of the extension of the secondary jack, in practice there is a jerk or hesitation in the upward movement of the load carrying device at the changeover from the operation of the primary jack to the operation of the secondary jack.